FIG. 13 is a sectional view of the neck of a cathode ray tube.
In the figure, a cathode ray tube 20 includes a neck 1. A free end of the neck 1 closed by a stem 2, and this neck 1 houses an electron gun 10. The electron gun 10 includes a heater 11, a cathode 12, first grid 13, second grid 14, third grid 15, which serves as a focus electrode, and fourth grid 16, these components being arranged in the stated order and held at specified intervals by insulating glass rods 17.
In for example the case of a 29" color cathode ray tube, when the cathode ray tube is operated, a high voltage of 28 kV is applied to fourth grid 16 of electron gun 10 via inner conducting film 3 and contactors 18 from an external anode button (not shown).
A high voltage of 6.7 kV is simultaneously applied to third grid 13 via a socket (not shown), stem pin 4a of third grid 15 and inner lead 19. Further, a voltage of about 700 V is applied to second grid 14, a voltage of about 150 V is applied to cathode 12, and a voltage of 0 V is normally applied to first grid 13.
Under these operating conditions, a potential difference of about 6 kV is set up between third grid 15 and second grid 14. If there is any burrs or flashes on the surface of that part of second grid 14 opposite to third grid 15 produced in the shaping (or forming) of the electrode or the manufacture of the electron gun 10, or if there is any dirt adhering to the interior of the cathode ray tube, stray emission of unwanted electrons may occur.
These unwanted electrons pass through the fourth grid 16, irradiate the surface of the cathode ray tube and cause it to fluoresce unnecessarily. This unnecessary fluorescence moreover occurs even when the screen is dark, and leads to a deterioration of image quality.
In order to prevent this emission of unwanted electrons, a voltage of four to five times the operating voltage of the cathode ray tube, i.e. a high voltage of about 30 kV, is applied from the outside, between stem pin 4a of third grid 15 and the other stem pins 4b, during the manufacture of the cathode ray tube 20 as shown in FIG. 14. When this high voltage treatment is applied, an arc discharge occurs between second grid 14 and third grid 15 which removes flashes, burrs, and dirt from second grid 14, and the emission of unwanted electrons is thereby suppressed.
As shown in FIG. 15, however, the stem pins 4b of second grid 14 and cathodes 12 are disposed around stem pin 4a of third grid 15 with a very small spacing from it. As a result, when a high voltage of four to five times the operating voltage was applied to third grid 15, a creeping discharge occurred between stem pins 4a and 4b, and a satisfactory high voltage treatment could not be achieved.
FIG. 16 is an enlarged perspective view of a structure of a silo-type base used to prevent such a creeping discharge. It comprises a silo-type base 30 wherein the stem pin 4a of the third grid is partitioned from other stem pins by walls which are attached to stem 2 via silicone rubber 31. A socket shown in FIG. 17 is used for connection with silo-type base 30 of FIG. 16 for the purpose of applying a voltage to the cathode ray tube from an external power source.
Using this silo-type base 30 and socket 32, the break-down voltage between stem pin 4a and the other stem pins 4b can be increased, but the high voltage treatment using four to five times the operating voltage could still not be performed with full satisfaction.
To solve the above problems, other high voltage treatments have been proposed, as disclosed for example in Japanese Patent Kokai Publication No. 101255/1979 wherein a high voltage is applied while maintaining at least stem 2 of cathode ray tube 20 in a high pressure gas atmosphere.
In this type of high voltage treatment, stem 2 of cathode ray tube 20 is enclosed in a sealed container 21, and a high pressure gas G is supplied to the container from outside as shown in FIG. 18, so that the threshold voltage at which creeping discharge begins on stem pins 4a and 4b is increased.
Using this method, the creeping discharge threshold voltage can be increased from about 23 kV in the conventional method to about 40 kV.
In the above method of high voltage treatment using high pressure gas, however, the creeping discharge threshold voltage was not always constant on production lines continuously manufacturing large numbers of cathode ray tubes, and creeping discharges sometimes occurred on the stem pins. In such cases, a satisfactory discharge did not occur between the second and third grid electrodes, dielectric characteristics were not consistent, and damage to the socket was caused by the energy of the discharging.